1. Field of the Invention
The present invention generally relates to an apparatus and method for controlling vehicle motion. More specifically, the invention relates to an apparatus for improving vehicle stability by controlling the brake torque of a vehicle during, for example, cornering.
2. Description of the Related Art
During vehicle motion such as cornering, both longitudinal forces (i.e., front to back) and lateral forces (i.e., side to side) influence the lateral and longitudinal behavior of the vehicle, as noted in the article "A Study On Vehicle Turning Behavior in Acceleration and in Braking", SAE Technical Paper No. 852184, pages 75-86, by Masato Abe. As further noted in the article, complicated equations of motion are involved in describing the combined lateral and longitudinal behavior of the vehicle, because many of the steady state equilibrium conditions which may exist during a constant speed mode of operation might not exist during vehicle braking or acceleration.
The varying longitudinal forces which affect vehicle stability during braking or acceleration have a tendency to cause the rear wheels of a vehicle to lock during braking due to a varying decrease in the rear wheel load. In order to prevent this rear wheel lock from occurring, some prior art brake control systems include a proportioning valve to adjust the amount of braking in proportion to the longitudinally changing loads of the front of the vehicle relative to the back of the vehicle.
Although the use of such a proportioning valve helps to prevent rear wheel lock from occurring during braking due to longitudinally changing load forces, it does not sufficiently adjust the braking action at the vehicle wheels to compensate for vehicle load changes that are due to lateral, i.e., side to side, forces. When a vehicle is undergoing a cornering maneuver, for example, there is not only a longitudinal load shift in a tangential direction to the vehicle's path of motion, but there is also a lateral load shift in a direction which is normal to the vehicle's path of motion. Such a lateral load shift is transferred, for example, from the wheels located on the inside of the curve in the vehicle's path to the wheels located on the outside of the curve in the vehicle's path. It is this lateral load shift which urges the vehicle out of its current path as defined by an existing radius of curvature, and into an oversteer or an understeer condition.
In the aforementioned article by Masato Abe, a study of the affect of acceleration and braking on vehicle turning behavior is presented. In this study, equilibrium equations of vehicle motion for constant lateral and longitudinal accelerations which describe the vehicle turning behavior during acceleration and braking are developed. The equations derived are used to obtain the radii of curvature of the vehicle path versus vehicle forward speed during constant acceleration or braking in turns. The vehicle turning behavior is also described by a characteristic line representing the lateral acceleration versus the longitudinal acceleration for a circular turning maneuver. For example, FIGS. 5-7 of the article reflect that for a given steering wheel angle, increased deceleration due to, for example, braking action (as reflected by negative acceleration in the FIGS. 5-7), results in a change from an understeer condition (i.e., an increase in turning radius), to an increasingly severe oversteer condition (i.e., a decrease in turning radius), with increased vehicle speed.
Although the prior art has recognized that longitudinal forces as well as lateral forces affect the vehicle motion during cornering, there is a need to provide a vehicle motion control system which will actually compensate for the lateral forces that detrimentally influence vehicle stability during the course of vehicle motion.